Catching Water That Would Otherwise Be Lost
The Okutsu Power Station Confluence Basin (Okutsu Hatsudensho Kairyuchi) exists for a single, thrifty purpose: to gather water that has already done its work and put it back to use. It sits just south of the main building of the Kamisaihara Power Station, and it collects two flows at once, the surplus overflow from that station's forebay tank and the discharge released after its turbines have run. Rather than let this water return uselessly to the river, the basin channels it onward into the Okutsu system, so the same water can serve a second time.
The structure itself is long and low: a reinforced-concrete channel running about 160 metres, with a recorded area of roughly 1,299 square metres. The walls that face the river are reinforced with granite laid in a diagonal, diamond-like pattern that Japanese masons call tanizumi. Built in 1930 (Showa 5), it is among the earlier elements of this cluster of structures in the Kamisaihara district of Kagamino, in the upper reaches of the Yoshii River.
A Name That Needs a Word of Explanation
Although it carries the name "Okutsu Power Station," the Confluence Basin is physically tied to the neighbouring Kamisaihara Power Station, whose surplus and tailwater it receives. The two stations form a linked cascade on the same stretch of river, and the basin is the hinge between them, taking Kamisaihara's leftover water into Okutsu's watercourse. The official record describes it as one of the water-conduit facilities of the Okutsu scheme, which is how a structure standing beside one plant comes to bear the name of another.
The basin was registered as a Tangible Cultural Property on 29 November 2006 under No. 33-0149, part of the same wave of listings that recognised the wider Okutsu and Kamisaihara structures. Its registration date falls a little later than several of its neighbours, which were listed that October, but the criterion is the same throughout: these works contribute to the historic character of the national landscape. The basin earns its place by showing how carefully the designers reused every available drop.
Reading a Long, Quiet Structure
Because it is stretched out along the river rather than built up in height, the basin is easy to overlook. Follow its length and the granite facing becomes the thing to watch: the diagonal coursing along the river-facing wall was chosen to withstand the current and hold the bank, and the interplay of straight concrete lines with the angled stonework rewards a slow look. This is engineering meant to be efficient rather than admired, which is part of why it feels honest.
The Kamisaihara district lies deep in the mountains of northern Okayama, upstream of the well-known Okutsu Onsen hot springs. The basin and the power-station buildings around it are working facilities on operational land, without visitor parking or interpretive signs. Anyone tracing the linked Okutsu and Kamisaihara structures should view them from public roads and keep clear of the equipment itself, which still carries the flows it was built to manage in 1930.
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Q&A
- What is a "confluence basin"?
- Here it is a long concrete channel that brings two flows together, the surplus overflow and the turbine discharge from the Kamisaihara Power Station, and passes the combined water into the Okutsu system so it can be reused.
- Why does an "Okutsu" structure sit at the Kamisaihara station?
- The two stations are linked along the same river. The basin belongs to the Okutsu water system but physically stands beside the Kamisaihara plant, receiving its leftover water, which is why it is recorded under the Okutsu name.
- How big is it, and when was it built?
- It is a reinforced-concrete structure about 160 metres long with an area of around 1,299 square metres, built in 1930. It was registered as a Cultural Property in November 2006 under No. 33-0149.
- What is the stone pattern on the walls?
- The river-facing walls are reinforced with granite set in a diagonal, diamond-like coursing known as tanizumi, chosen to resist the current and protect the bank.
- Can it be visited?
- It is a working facility on operational land with no visitor amenities, so it should be viewed from public ground as part of the wider group of structures along the river.
Basic Information
| Name | Okutsu Power Station Confluence Basin (Okutsu Hatsudensho Kairyuchi) |
|---|---|
| Location | Kamisaihara, Kagamino, Tomata District, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (Structure), No. 33-0149 |
| Registered | 29 November 2006 |
| Built | 1930 (Showa 5) |
| Quantity / Scale | 1 site; reinforced concrete, area approx. 1,299 m² (length approx. 160 m) |
| Owner | The Chugoku Electric Power Co., Inc. |
| Access | Working infrastructure; no public entry. View from public ground only. |
References
- National Database of Cultural Properties (ACA) — Okutsu Power Station Confluence Basin
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00006060
- Cultural Heritage Online — Okutsu Power Station Confluence Basin
- https://bunka.nii.ac.jp/heritages/detail/152665
- Chugoku Electric Power — Hydroelectric Stations
- https://www.energia.co.jp/energy/general/water/water_all.html
- Kagamino Town official site
- https://www.town.kagamino.lg.jp/
Last verified: 2026.07.04